1. Field of the Invention
This invention relates to the accurate alignment and assembly of sensors and signal processing circuits in a sensor assembly and, more particularly, to a sensor assembly for use with an electronic printer to control the start of plot and print lines.
2. Description of the Prior Art
Marks printed on a print medium have been used to direct the start of plot in a print process where more than one print head is required to provide the printed image. Because of the stringent requirement of high resolution printing, in which an error of 0.025", for example, is not tolerated, the correct positioning and alignment of the print medium with the print heads and of the print heads with the sensors which control start of plot and the print lines are particularly important.
In one embodiment, the alignment of the print medium with respect to the print heads is accomplished by sensing both a printed tracking line which contains a set of special marks called start-of-plot marks (SOP marks), and a continuous line of light and dark bands called "tick marks." One such system is described in U.S. Pat. No. 5,047,631 to D. Frese, entitled "Self-Referencing Start of Plot System and Method," issued Sep. 10, 1991; U.S. Pat. No. 4,047,631 is hereby incorporated by reference in its entirety.
In the prior art, the tracking line is sensed by a single lens optical system 42 as shown in FIGS. 4A and 4B. In FIG. 4A, the print medium 128 containing tick mark track 720 and tracking line 110, which also includes SOP mark 211, moves in the direction of 129 over optical system in sensor assembly 42, with the tracking line 110 directly above window 42e of optical system 42. Print medium 128 is illuminated by two infrared emitting diodes (IRED) 42c and 42c' (FIG. 4B) located off axis from the tracking line. An optical element 42d is required to project the image of the tracking line 110, as illuminated by IRED 42c and 42c', onto two photo-voltaic cells 42a and 42b which sense the location of the tracking line 110 and the start of plot mark 211. While this system worked, it required a precisely aligned optical element 42d and highly accurate control over the distance between the print medium and the locations of each of the photovoltaic cell 42a and 42b. The assembly cost is therefore high, and the resulting sensor assembly is also relatively large to be undesirable in a printer where space is a premium.
Also in the prior art, the sensors must be accurately aligned to "windows"--precisely defined areas such as 42e on the sensor assembly through which light is transmitted to the sensors. Circuit boards containing the sensors had to be individually and accurately positioned with the sensor assembly during the assembly process to achieve the necessary alignment of the windows to the sensors. This is an expensive step in the assembly process.
Another problem relates to the packing of the signal processing circuits into the sensor assembly. Since the sensor assembly must be made as compact as possible, the circuits for processing the sensor responses must be split up and separately mounted on more than one circuit board, requiring routing and interboard wiring to transmit signals and power. Signals and power may be routed by providing individual jumpers or connections. However, providing individual jumpers to connect terminals of the circuit boards is labor intensive and using connectors to connect circuit boards is space inefficient as well as expensive.